In recent years, attention has been paid to renewable biological fuel such as bioethanol (biomass ethanol) because global warming is being recognized as a serious issue and cost of fossil fuel is rising.
The bioethanol is produced by fermenting and distilling biomass such as sugarcane or corn. However, if water is mixed into ethanol, combustion efficiency deteriorates. Due to this, it is necessary to manage a water content strictly in a process of producing the bioethanol.
Patent Document 1 discloses a method of measuring a water content in a nonaqueous solvent using conductive polymer which electric conductivity reversibly increases when the conductive polymer is hydrated using water. However, with the method disclosed in the Patent Document 1, if an electrolyte other than the water is mixed into the nonaqueous solvent as impurity, measurement error tends to occur due to influence of mixture of the electrolyte.
Furthermore, as a method of measuring a water content in an organic solvent or the like, there is known Karl-Fischer method of measuring a water content using a Karl Fischer reagent containing iodine, sulfur dioxide, a base, alcohol and the like and reacting with water selectively and quantitatively. While there are known, as types of the Karl-Fischer method, coulometric titration and volumetric titration, apparatuses for the both methods are large in size and complicated.
Moreover, the coulometric titration that is an electrochemical method out of the types of the Karl-Fischer method is intended to indirectly measure a water content by adding a sample to an electrolytic solution containing the Karl-Fischer reagent, subjecting the sample-added electrolytic solution to electrolytic oxidation, calculating a quantity of electricity required for the electrolytic oxidation based on Faraday's law from a quantity of the iodine generated according to a reaction represented by a formula “2I−-2e→I2”, and reducing the quantity of electricity to the water content according to “1 mg of water=10.71 coulomb”.
Patent Document 1: Japanese Unexamined Patent Publication No. 1994-160319